This proposal concerns the basic study and the preclinical development of the ansamycin antibiotics as drugs that inhibit pathways important for the growth of advanced breast cancer. Ansamycins are a novel class of antitumor antibiotics that bind to Hsp90 and cause the degradation of several key signaling molecules, including HER2, and the inactivation of the Akt kinase signaling pathway. We have shown that ansamycins lead to RB-dependent G1 arrest and subsequent apoptosis of cancer cells and that they have antitumor activity in animal models at doses that are tolerable to the host. Cancer cells that overexpress HER2 are particularly sensitive. These properties suggest that ansamycins may be useful in the therapy of breast cancer and have led to an NCI-initiated phase I trial of 17-allyl-aminogeldanamycin (17-AAG) in which we are taking part. The current application proposes studies aimed at determining the mechanisms whereby ansamycins cause growth arrest and differentiation of breast cancer cells that overexpress HER2, evaluating the role of inhibition of Akt kinase activity in these processes, and establishing how 17-AAG sensitizes cancer cells to cytotoxic agents. In collaboration with other investigators in this program project, findings from tissue culture models will be validated using xenograft and genetic murine tumor models of breast cancer. The long-term goals of this project are to understand the role of Hsp90-regulated pathways in breast cancer, to develop a mechanistic basis for the design of clinical trials of 17-AAG, and to establish tissue culture and animal models for the development of signal transduction inhibitors as cancer drugs.